This invention relates to a process for separating particles suspended in a fluid; it also relates to an improved device for the separation of such particles, whether solid, liquid, or gaseous.
This invention has potential applications in many fields. It will be particularly suitable to the paper industry, without being limited thereto. More specifically, it will be useful for the treatment of solutions containing suspended particles, such as: fibrous suspensions from waste papers; paper pulps to be cleaned; effluent water from a paper machine in which separate collection of the fibers or fillers, waste water, and the like is desirable.
In the description, the invention is more specifically described in the context of applications relating to the paper industry. It shall be understood that this specific embodiment has only been chosen as an illustrative example. In fact, the invention may have other applications in situations which require classification or fractionation performed by centrifugal means such as the recovery of nonmiscible liquids of different specific gravities, within mixtures, and the like. Likewise, although the fluid is usually liquid, such as water, it may also be gaseous.
Solutions with suspended particles, which are handled in the paper industry, contain the following main elements in relative proportions which vary a great deal:
a fluid carrier (usually an aqueous solution),
natural, artificial, or synthetic fibers, more or less individualized,
solid particles which vary a great deal in size and specific gravity (mineral contents, miscellaneous impurities: hot melt, plastics, inks, adhesives, tars, metallic particles, sand, and the like) which shall be referred to as "contaminants",
and also, liquid or gaseous particles (air).
The cleaning process for a suspension involves separation of one or several fractions of undesirable particles from the said suspension, and in the particular case of paper making, recouperation of a fibrous suspension free from contaminants which are detrimental in future recycling.
Among the procedures presently known for such treatment of suspensions, the most commonly used are based on the principle of separation according to the differences in the particles' specific gravity or size. In these procedures the suspension is introduced into a revolution chamber where it flows in a vertical motion called a "vortex". The particles in the suspension are therefore subjected to the simultaneous action of two forces:
the centrifugal force, resulting from the centrifugal acceleration acting on the mass of each particle and tending to draw such particle from the center outward;
and the centripetal force, due to the action of the radial pressure gradient on the volume of the particle tending to draw such particle towards the center of the vortex.
If the specific gravity of the particle is equal to that of the fluid carrier, these two forces balance each other and there is no average radial motion of the particle with respect to the fluid.
However, if the suspended particle has a specific gravity less than that of the fluid carrier, the effect of the radial pressure gradient is higher than the effect of the centrifugal force. In that instance, the particle moves toward the longitudinal axis of the vortex. On the other hand, if the particle has a specific gravity greater than that of the fluid carrier, the effect of the radial gradient is lower than the effect of the centrifugal force and the particle moves to the periphery of the revolution chamber.
To make the description easier to follow, the terms "light particles" or "light components" shall represent particles of a density lower than that of the fluid carrier, while terms "heavy particles" or "heavy components" represent those of a density higher than that of the fluid carrier.
The components whose density is either lower or higher than that of the fluid carrier, but whose migration velocity is low essentially due to their very small size, both constitute a fraction hereafter designed "intermediate fraction".
Devices called "hydrocyclones" or "centricleaners", which are made of a stationary conical chamber, have been suggested. In these devices, the suspension to be cleaned is tangentially introduced at the head of the conical chamber; the heaviest particles are removed at the opposite end; and the solution thus cleaned then is collected at the head of the chamber close to the longitudinal axis.
These hydrocyclone devices have usually proved efficient for separating the heavy particles (sand, metallic particles, and the like) but have given poor results for separating light particles, especially those of a density close to that of the fluid carrier.
In fact, in a cleaning apparatus of the hydrocyclone type with a stationary wall, the only adjustable operational parameter for a given device is the tangential velocity of inflow of the suspension. In order to eliminate the heavy particles at the periphery, this velocity must be maintained at a rather high rate, which causes a rapid flow through the central portion of the apparatus, and therefore does not permit a sufficient amount of time for the desired dissociation of the light particles. Accordingly, nearly all of the light particles are found in the "cleaned suspension".
Furthermore, the use of high velocities imparts throughout the suspension a very high turbulence level which counteracts the effect of separation between the various particles.
In order to facilitate the elimination of the light particles in this type of apparatus, it has been suggested that a small diameter plunging tube be placed in the center of the vortex in order to remove the low density contaminants. The proportion of light contaninants thus separated, however, remains rather low, due to the reduced size of the central zone of the vortex.
In the cases of suspended solutions where the contaminants to be removed are mainly light ones, it has also been suggested that the suspension be tangentially injected into a cylindrical chamber having stationary walls and also tangentially removed while the contaminants are extracted axially. In this type of apparatus, the solution flows at a much lower velocity resulting in a low radial pressure gradient throughout the solution which does not permit the elimination of the lightest particles.
A cleaning device is described in French Pat. Nos. 2,091,170, and 2,293,983, which tried to use a large driving force and avoid the problem of turbulence. This device, aiming at meeting theoretical forced vortex conditions as much as possible, is comprised of two concentric cylindrical walls rotating in synchronism. In operation, the suspension is introduced into the annular space thus formed between the concentric walls and flows through it in such a manner that the suspension and the walls rotate together as one unit. However, the efficiency of this device is limited by the effect of the concentration of the suspension to be cleaned due to the absence of agitation which results in a rapid clogging-up of the device. Furthermore, in the case of solutions with suspended fibers, the morphology of the fiber components in the suspension creates an additional impediment affecting the operating efficiency of this device. In the absence of agitation, the fiber components tend to rapidly aggregate into a coherent network which "traps" the contaminants and prevents them from moving within the fluid.
U.S. Pat. No. 1,712,184 describes a forced vortex system with rotating divergent walls, wherein the solution is introduced through the bottom and flows into the area of reduced pressure created by the wall's rotation. Due to the divergence of the wall, the velocity of the suspension is always lower than that of the wall. This significantly limits the efficiency of the separation, and therefore does not permit controlling the time the solution remains inside the device independently of the velocity of rotation. In practice, this device lacks versatility to the extent that it only permits variation of the velocity of rotation.
In the Australian Pat. No. 465,775, a classic hydrocyclone is described wherein its wall is made to rotate in order to superimpose a forced vortex upon the free vortex created in the hydrocyclone. Here the suspension is tangentially introduced with an angular velocity higher than that of the wall. However, here the majority of the heavy particles are collected approximately along the longitudinal axis of the hydrocyclone, and the light particles are also collected along the same axis. A loss of the suspension's kinetic energy of rotation results therefrom since most of the suspension to be recovered is recovered axially and all the fluid kinetic energy is dispersed within the vortex. Furthermore, the rotating outlet device acting like a pump, when operating results in an additional amount of energy consumption. On the other hand, as mentioned before, the described embodiment does not make it possible to have a large centrifuging zone for the particles in the center of the vortex, since all the cleaned suspended solution is collected near the longitudinal axis.
If the device of Australian Pat. No. 465,775 were modified to be cylindrical instead of conical (this hypothesis was not described however), the heavy components could be collected at the periphery while the bulk of the suspended solution could be recovered at the center. This modification presents some disadvantages similar to those mentioned in the above embodiment. Again, it would be impossible to recover the kinetic energy of rotation and the benefit of the favorable effects of a large zone of centrifugation in the central portion of the vortex, for the elimination of the heavy components.